High Resistance Grounding (HRG) on 4.16kV System 2

[rockman7892]

Are there any issues with applying an HRG on MV system at 4.16kV? I'm used to seeing HRG reserved for LV (480V) system with MV systems typically having LRG.

With an HRG on an MV system is it typically applied at the same 10A range as on LV systems (assuming that is above system charging currents)?

Can the system continue to stay in service with a ground fault similar to LV systems or must the ground fault be removed via localized tripping on MV systems?

Do these HRG systems on MV typically have the same pulsing detection system with dedicated CT's / ammeters on feeders to aid in ground fault location?

 

[dpc]

HRG is pretty common at 4160 V, especially if there is generation. 10 A is at the extreme upper limit for HRG at MV. More like 3 to 5 amps.

The downside is lack of fault selectivity. Generally the entire system is tripped off for a ground fault. In theory, operation can continue with one phase grounded, but I've personally never encountered that approach at MV. Grounding is done via a small single-phase distribution transformer with the resistor on the low voltage side - usually 240 or 120 V.

Faults generally detected via a voltage relay across the resistor.

Advantage over LRG is decreased damage in the event of a ground fault, especially to generators, motors, and transformers.

Cheers,

Dave

 

[rockman7892]

Thanks dpc

I've seen HRG with MV generation but mostly where gen's are unit connected through GSU so not the issues with fault selectively like you pointed out. I'm just not used to seeing in an industrial MV unit substation application where fault selectivity is likely required.

Is there a MV voltage threshold at which HRG can be used on MV systems? I recall seeing somewhere that corona becomes an consideration/issue when applying above 15kV?

It is also my understanding that only up to a maximum of 10A is allowed to flow continuously during a fault without having to trip offline. Above 15kV the system charging current becomes greater than 10A which does not make it practical to keep system running continuously and thus system must be tripped offline?

 

[3I0]

Commonly medium voltage grounding resistors are not sized for continuous operation, at least in the vast majority of the industrial facilities I have worked in.

In the typical industrial distribution substation I have seen, fault selectivity is accomplished on high resistance grounded systems by using a combination of core balance CTs on all feeders and a CT on the transformer neutral, then using definite time overcurrent elements to provide time grading as you get progressively closer to the source.

Selectivity on high resistance grounded systems can get tough with multiple sources of ground fault current connected together if the fault is not downstream of a feeder with a core balance CT, and/or if the fault location is inside of a source (for example parallel generators with a stator ground fault in one of them).

 

[RRaghunath]

Applying HRG in MV system is unusual.
Main issue is with the system capacitance - capacitance of all the cables and equipment connected in downstream feeders put together. This makes the task of limiting the current to below 10A impractical.
IEEE also mentions LRG for MV system.
Having said that HRG can be applied selectively if the downstream cable lengths are limited / overall capacitive currents are well below 10A.
Protection Selectivity could be achieved to some extent by having core balance CTs in individual outgoing feeders, I suppose.

 

[LionelHutz]

Supplying equipment mostly in the 5kV to 15kV range, I see HRG and solidly grounded both about equally with LRG being a small percentage of sites. Typically in the 5A range. Putting zero sequence CT detection in each piece of switchgear gives the ability to selectively trip.

 

[dpc]

In my experience, for industrial systems, solidly grounded MV systems are not common and never recommended. LRG historically have been the standard approach with fault currents limited to anywhere from 100 A up to 1000 A. Solidly grounding MV industrial systems is contrary to IEEE recommendations, last time I checked. HRG at MV is the standard for MV generators in the US least. I agree that for large systems with long cable runs, the capacitance of the system can be a limiting factor for HRG. I remain a skeptic regarding selective coordination with HRG, but I'm old and set in my ways.